This application claims the benefit of Japanese Application Nos. 10-104885, 10-120383, 10-206249 and 10-226142.
1. Field of the Invention
The present invention relates to a constant velocity joint and a wheel-support rolling bearing unit incorporating the constant velocity joint.
A wheel-support rolling bearing unit according to the present invention is a so-called fourth-generation hub unit, and utilized for supporting the drive wheels {(which imply front wheels of an FF car (front-engine front-drive car), rear wheels of an FR car (front-engine rear-drive car) of an RR car (rear-engine rear drive car), and whole wheels of 4WD car (four-wheel drive car) } held on the independent suspension so that the drive wheels are rotatable about the suspension.
A constant velocity joint according to the present invention is integrally incorporated into a rolling bearing unit for supporting drive wheels on, e.g., an independent suspension, and utilized for transmitting a driving force from a transmission to the drive wheels.
2. Related Background Art
A constant velocity joint is provided between a transmission of an automobile and a drive wheel supported on an independent suspension, whereby a driving force (traction) of an engine is transmittable to the drive wheel at the same angular speed along an entire periphery irrespective of a relative displacement between a differential gear and the drive wheel and of a steering angle given to the wheel. What has hitherto been known as the constant velocity joint used for such a mechanism, is disclosed, e.g., U.S. Pat. No. 3,324,682, U.S. Pat. No. 3,412,580 and U.S. Pat. No. 4,589,857.
This type of constant velocity joint 1 which has been known so far is constructed so that a rotary force is, as shown in, e.g., FIGS. 21-23, transmitted between an inner race 2 and an outer race 3 through six pieces of balls 4, 4. The inner race 2 is fixed to an external side end (a left side end in FIG. 21) of one shaft 5 rotationally driven by the transmission. Further the outer race 3 is fixed to an internal side end (a right side end in FIG. 21) of another shaft 6 for fixing the drive wheel. Six streaks of inner engagement grooves 7, 7 each taking a circular arc shape in section are formed in an outer peripheral surface 2a of the inner race 2 in a direction right-angled to a circumferential direction at an equal interval in the circumferential direction. Six streaks of outer engagement grooves 8, 8 each taking the circular arc shape in section are likewise formed in an outer peripheral surface 3a of the outer race 3 in positions facing to the inner engagement grooves 7, 7 in the direction right-angled to the circumferential direction.
A cage 9 assuming a circular arc shape in section but an annular shape on the whole is sandwiched in between the outer peripheral surface 2a of the inner race 2 and the inner peripheral surface 3a of the outer race 3. Pockets 10, 10 are formed in positions aligned with the two groups of inner and outer engagement grooves 7, 8 as well as in six positions in the circumferential direction of the cage 9, and totally six pieces of balls 4, 4 are held one by one inwardly of each of the pockets 10, 10. These balls 4, 4 are capable of rolling along the two groups of inner and outer engagement grooves 7, 8 in a state of being held in the pockets 10, 10.
The pockets 10, 10 are, as illustrated in FIG. 23, each takes a rectangular shape elongated in the circumferential direction, and structured to, even when a spacing between the balls 4, 4 adjacent to each other in the circumferential direction might change with a variation in an axial crossing angle a which will hereinafter be explained, absorb this change. Namely, a positional relationship between bottom surfaces 7a, 7a of the inner engagement grooves 7, 7 and a positional relationship between bottom surfaces 8a, 8a of the outer engagement grooves 8, 8, become such as a relationship of the longitude lines on a globe as indicated by the one-dotted chain line in FIG. 24. If the central axis of the inner race 2 is concentric with the central axis of the outer race (the axial crossing angle xcex1=180xc2x0), each of the balls 4, 4 exists in the vicinity of a position corresponding to the equator on the globe which is indicated by the two-dotted line in FIG. 24. Whereas if the central axis of the inner race 2 is not concentric with the central axis of the outer race (the axial crossing angle xcex1 less than 180xc2x0), the balls 4, 4 displace in reciprocation (displace alternately in the direction of the North Pole and in the direction of the South Pole on the globe) in the up-and-down direction in FIG. 24 with a rotation of the constant velocity joint 1. As a result, the spacing between the balls 4, 4 adjacent to each other in the circumferential direction changes, and hence the pockets 10, 10 each takes the rectangular shape elongated in the circumferential direction, thereby enabling the spacing therebetween to change. Note that the bottom surfaces 7a, 7a of the inner engagement grooves 7, 7 and the bottom surfaces 8a, 8a of the outer engagement grooves 8, 8, are not concentric with each other as obvious from the explanation which follows. Accordingly, the lines corresponding to the longitude lines exist in positions slightly deviating from each other for every corresponding engagement groove 7 or 8.
Further, as shown in FIG. 21, the balls 4, 4 are disposed within a bisection plane c which bisects the axial crossing angle xcex1 between the two shafts 5, 6, i.e., the angle xcex1 made by two lines a and b at a point-of-intersection O between a central line a of one shaft 5 and a central line b of the other shaft 6. Therefore, the bottom surfaces 7a, 7a of the inner engagement grooves 7, 7 are located on a spherical surface wherein a point d existing away by h from the point-of-intersection O on the central line a is centered, and the bottom surfaces 8a, 8a of the inner engagement grooves 8, 8 are located on a spherical surface wherein a point e existing away by h from the point-of-intersection o on the central line b is centered. The outer peripheral surface 2a of the inner race 2, the inner peripheral surface 3a of the outer race and two inner and outer peripheral surfaces of the cage 9, are, however, located on the spherical surface with the point-of-intersection O being centered, thereby enabling the outer peripheral surface 2a of the inner race 2 and the inner peripheral surface of the cage 9 to slide on each other, and also the outer peripheral surface 3a of the outer race 3 and the outer peripheral surface of the cage 9 to slide on each other.
In the case of the thus constructed constant velocity joint 1, when the inner race 2 is rotated by one shaft 5, this rotary motion is transmitted via the six balls 4, 4 to the outer race 3, whereby the other shaft 6 rotates. If a positional relationship (which implies the axial crossing angle a) between the two shafts 5, 6 changes, the balls 4, 4 roll along the two groups of inner and outer engagement grooves, thus allowing the displacement between one shaft 5 and the other shaft 6.
The basic structure and operation of the constant velocity joint are as described above. The basic structure and operation of the constant velocity joint which have been explained referring to FIG. 21 are applied to the present invention and the embodiments thereof which will be discussed later on.
On the other hand, it has been a technical pursuit over the recent years that the constant velocity joint described above is combined integrally with a wheel-support rolling bearing unit for rotatably supporting the wheel on a suspension. Namely, the operation of rotatably supporting the wheel of an automobile on the suspension involves the use of the wheel-support rolling bearing unit in which the outer race and the inner race are rotatably combined through rolling members. If the thus constructed wheel-support rolling bearing unit is combined integrally with the above-described constant velocity joint, the wheel-support rolling bearing unit and the constant velocity joint can be so constructed as to be downsized and to reduce weights thereof on the whole. What has hitherto been well known as the wheel-support rolling bearing unit, i.e., a so-called fourth-generation hub unit structured to integrally combine the wheel-support rolling bearing unit with the constant velocity joint, is disclosed in Japanese-Patent Application Laid-Open Publication No.7-317754.
FIG. 25 shows a prior art structure disclosed in the same Publication. An outer race 11, which does not rotate in a state of being supported on the suspension as well as in a state of being assembled to a vehicle, includes a first fitting flange 12, formed on an outer peripheral surface thereof, for supporting the wheel on the suspension, and plural trains of outer race tracks 13, 13 formed along an inner peripheral surface, respectively. A hub 16 constructed by combining first and second inner race members 14, 15 is disposed inwardly of the outer race 11. The first inner race member 14 of these first and second inner race members 14, 15 is formed in a cylindrical configuration and includes a second fitting flange 17, provided at a portion, closer to one side end (on a left side in FIG. 25), on the outer peripheral surface, for supporting the wheel, and a first inner race track 18 provided at a portion closer to the other side end (on a right side in FIG. 25), respectively. While on the other hand, the second inner race member 15 includes a cylindrical portion 19, provided at one side end (a left side end in FIG. 25), for externally fixedly fitting the first inner race member 14, an outer race 3A for a constant velocity joint 1a, which is provided at the other side end (a right side end in FIG. 25), and a second inner race track 20 formed in an outer peripheral surface of an intermediate portion. Then, a plurality of rolling members 21 and another plurality of rolling members 21 are provided between the outer race tracks 13, 13 and the first and second inner race tracks 18, 20, whereby the hub 16 is rotatably supported inwardly of the outer race 11.
Further, engagement grooves 22, 23 are formed in positions aligned with each other on the inner peripheral surface of the first inner race member 14 and on the outer peripheral surface of the second inner race member 15, and a stop ring 24 is provided in a state of bridging the two engagement grooves 22, 23, thus preventing the first inner race member 14 from coming off the second inner race member 15. Further, a portion between an outer peripheral edge of one side end surface (a left side end surface in FIG. 25) of the second inner race member 15 and an inner peripheral edge of a stepped portion 25 formed on the inner peripheral surface of the first inner race member 14, is welded 26, thereby fixedly joining the first and second inner race members 14, 15 to each other.
Moreover, covers 27a, 27b each taking substantially a cylindrical shape and composed of a metal such as a stainless steel etc and annular seal rings 28a, 28b each composed of an elastic material such as elastomer like a rubber, are provided between openings formed at both side ends of the outer race 11 and the outer peripheral surface of the intermediate portion of the hub 16. The covers 27a, 27b and the seal rings 28a, 28b cut off the portions provided with the plurality of rolling members 21, 21 from outside, thereby preventing grease existing in those portions from leaking outside and also preventing foreign matters such as rain water and dusts etc from permeating those portions. Moreover, a screen board 29 for closing the inside of the second inner race member 15 is provided inwardly of the intermediate portion of the second inner race member 15, thereby ensuring a rigidity of the second inner race member 15 and preventing the foreign matters from arriving at the constant velocity joint 1a, which have entered the interior of the second inner race member 15 from an opening at the front side end (a left side end in FIG. 25) of the second inner race member 15. Note that the constant velocity joint 1a is constructed in the same way as that of the constant velocity joint 1 previously illustrated in FIGS. 21-23.
When assembling the thus constructed wheel-support rolling bearing unit to the vehicle, the outer race 11 is supported through the first fitting flange 12 on the suspension, and the wheel defined as a drive wheel is fixed through the second fitting flange 17 to the first inner race member 14. Further, a front side end of an unillustrated drive shaft rotationally driven by an engine through a transmission, is spline-engaged with the inside of the inner race 2 constituting the constant velocity joint 1a. When the automobile travels, rotations of this inner race 2 are transmitted via the plurality of balls 4 to the hub 16 including the second inner race member 15, thereby rotationally driving the wheel.
For attaining further downsizing of the fourth-generation wheel-support rolling bearing unit described above, it is effective to reduce a diameter of a circumscribing circle of each of the plurality of balls 4, 4 constituting the constant velocity joint 1a. Then, the diameter of each of the balls 4, 4 is reduced for decreasing the diameter of the circumscribing circle, and besides it is required for securing a torque transmittable through the constant velocity joint 1a that the number of the balls 4, 4 be increased. Moreover, under such circumstances, even when increasing the number of the balls 4, 4, there might be a necessity for ensuring strength and durability of each of column members 30, 30 (see FIGS. 22, 23, 27 and 29 to 31) existing between the plurality of pockets 10, 10 provided in the cage 9 in order to secure a durability of the cage 9 for holding the respective balls 4, 4.
The reason why when the number of the balls 4, 4 is increased from 6 up to 8, there rises a rate of the balls occupying the cage in the circumferential direction even if a major diameter Da is reduced to some extent. As a result, a circumference-directional width of each of the column members 30, 30 (FIGS. 22 and 23) existing between the pockets 10, 10 adjacent to each other in the circumferential direction, is narrowed, and there is a deficiency in terms of a rigidity of the cage 9, which might lead to a possibility in which damages such as cracks etc occur at a peripheral edge of each of the pockets 10, 10 with a long-term use. Namely, if the constant velocity joint 1a is operated in a state of giving a joint angle (at which a positional relationship between the central axis of the inner race 2 and the central axis of the outer race 3A deviates from a rectilinearity, i.e., a supplementary angle of the axial crossing angle xcex1 shown in FIG. 21), the respective balls 4, 4 receive forces as indicated by arrowheads a, a in FIGS. 26 and 27 from the bottom surfaces 7a, 8a of the two inner and outer engagement grooves 7, 8. Then, the balls 4, 4 are pressed by a resultant force of the forces indicated by the arrowheads a, a against an intermediate portion of an inner surface of the rim portion 31 of the cage 9. As a result, a moment load, with a connecting portion to the column members 30, 30 being centered, is applied to the rim portion 31, and a stress is applied to this connecting portion. This stress becomes greater as a length of each of the pockets 10, 10 in the circumferential direction becomes larger, and as the length dimension of each of the column members 30, 30 in the circumferential direction becomes smaller, with the result that the connecting portion is easily damaged like cracks etc. Such being the case, it is required for ensuring the ample durability of the cage 9 that the length dimension of each of the pockets 10, 10 in the circumferential direction be reduced, and that the length dimension, in the circumferential direction, of each of the column members 30, 30 adjacent to each other in the circumferential direction be increased.
The process of increasing the length dimension of each of those column members 30, 30 is controlled in terms of preventing interference with the balls 4, 4. To be more specific, first, the length of each of the pockets 10, 10 in the circumferential direction needs, when rotating the constant velocity joint 1a in the state of giving the joint angle, to be large enough to enable each of the balls 4, 4 to displace in the circumferential direction of the cage 9. Second, the above length must be, after assembling together the inner race 2, the outer race 3A and the cage 9 in order to assemble the constant velocity joint la, large enough to incorporate the balls 4, 4 into the pockets 10, 10 of the cage 9.
European Patent 0 802 341 A1 discloses the constant velocity joint 1b as shown in FIGS. 28-31 by way of a structure for increasing the length dimension of each of the column members 30, 30 while setting the number of the balls 4, 4 to 6 or larger in consideration of the above point. The constant velocity joint 1b disclosed in the above Publication is structured to transmit the rotary force between the inner race 2 and the outer race 3 through eight pieces of balls 4, 4. Then, in the case of the structure disclosed in the same Publication, two types of pockets 10a, 10b each having a different length dimension in the circumferential direction, are disposed alternately at an equal interval in the circumferential direction. With this arrangement, as compared with the case of using the single type of pockets, it is feasible to increase a circumference-directional width of each of the column members 30, 30 existing between the pockets adjacent to each other in the circumferential direction. There is made, however, no contrivance about the width of each of the column members 30, 30 in terms of securing the durability of the cage 9a while ensuring life-spans of other components of the constant velocity joint 1b. 
In other words, there is made no contrivance of optimally controlling a relationship between the major diameter of each of the balls 4, 4 constituting the constant velocity joint 1b and the width of each of the column members 30, 30, considering a relationship between the rolling fatigue line-span of each of the inner and outer engagement grooves 7, 8 and the strength of the cage 9a. The above Publication does not disclose such a point at all that the constant velocity joint 1b is designed in consideration of the above point.
In the case of the above-described structure disclosed in the European Patent 0 802 341 A1, each of the balls 4, 4 is held in each of the pockets 10a, 10b, and hence it is difficult to equilibrate at a high level the major diameter and the number of the balls 4, 4 and the length dimension of each of the column members 30, 30 when ensuring these factors. Therefore, the constant velocity joint capable of transmitting sufficiently a large torque and exhibiting an enough durability can not be necessarily actualized.
It can be considered to enlarge a section area of each of the column members 30, 30 by increasing a thickness of the cage 9 for securing the strength and the durability thereof even when the width of each of the column members 30, 30 is small.
There arises, however, a fresh problem which follows, if the major diameter of the cage is increased or if a minor diameter thereof is decreased in order to enlarge the sectional area.
First, the increase in the major diameter of the cage leads to a rise in a diameter of an inner peripheral surface 3a of the outer race 3 (3A). This rise in the diameter of the inner peripheral surface 3a leads to a decrease in depth of the outer engagement groove 8. Similarly, a decrease in the minor diameter of the cage leads to a reduction in a diameter of an outer peripheral surface 2a of the inner race 2. This decrease in the diameter of the outer peripheral surface 2a leads to a decrease in depth of the inner engagement groove 7.
When the depth of each of the two groups of outer and inner engagement grooves 8, 7 decreases, there is lessened the rigidity of the constant velocity joint 1 (11a) in a rotational direction, which is based on an engagement of each of the balls 4, 4 with each of the two groups of engagement grooves 8, 7. Further, when transmitting a large torque between the inner race 2 and the outer race 3 (3A), a rolling surface of each ball 4 becomes easier to run on an opening edge of each of the engagement grooves 8, 7. As a result, the durability of the constant velocity joint is ensured with the difficulty because of a shortened rolling fatigue life-span of the rolling surface of each ball 4, and so forth.
Accordingly, it must be controlled in terms of obtaining a required depth of the engagement groove that the major diameter of the cage 9 is increased or that the minor diameter thereof is reduced.
On the other hand, it is also required that a minimum thickness of the cage be controlled in terms of ensuring the durability of the constant velocity joint 1 (1a). Namely, if the cage 9 is composed of a material having a large strength such as, e.g., a high-function resin and a high-tension steel etc., the strength and the durability of the column member 30 itself can be ensured. In this case also, however, if the thickness thereof is too small, the following problem might arise.
That is, as obvious from the discussion given above, during an operation of the rzeppa type constant velocity joint 1 (1a) at which the present invention aims, the balls 4, 4 displace in the diametrical direction of the cage 9 as well as in the circumferential direction thereof. With such a displacement, when a maximum-major-diameter portion of the ball 4 impinges upon the opening edge of the pocket holding the ball inside, there might be a possibility wherein this opening edge is chipped off.
To begin with, if the major diameter of the cage 9 is too small, the maximum-major-diameter portion of the ball 4 existing upward in FIG. 21 impinges upon the peripheral edge of the opening on the side of the major diameter of the pocket 10. Whereas if the minor diameter of the cage 9 is too large, the maximum-major-diameter portion of the ball 4 existing downward in FIG. 21 impinges upon the peripheral edge of the opening on the side of the minor diameter of the pocket 10. Every opening peripheral edge of the pocket 10 takes a sharp configuration and might be therefore, if strongly pressed by the rolling surface of the ball 4, chipped into minute fragments. A sectional configuration of the opening peripheral edge on the side of the major diameter has an acute angle especially when the pocket 10 is formed by punch-out working, and hence, if the cage 9 is composed of a steel subjected to a hardening process, the above chips might be easily produced.
Then, if the chips enter between the balls 4, 4 and the engagement grooves 8, 9, the inner surfaces of the engagement grooves 8, 7 and the rolling surfaces of the balls 4, 4 are damaged, which in turn causes a decline of the durability of the constant velocity joint 1 (1a).
Accordingly, it must be controlled in terms of preventing the rolling surfaces of the balls 4, 4 from impinging upon the opening peripheral edges of the pockets 10 to reduce the major diameter of the cage 9 or to increase the minor diameter thereof.
As described above, it is required that the maximum and minim of the major and minor diameters of the cage 9 be controlled in terms of securing the rigidity and the durability of the constant velocity joint 1 (1a), however, no contrivance on this point has been made in the prior art.
Further, the specification of British Patent No. 1,537,067 discloses a structure in which the balls 4, 4 are, as shown in FIG. 32, held by twos in each of three pockets 10c, 10c formed in positions at an equal interval in the circumferential direction of a cage 9b. According to this structure, a length dimension of each of column members 30, 30 existing between the pockets 10c, 10c adjacent to each other in the circumferential direction, is increased corresponding to a degree to which the interval between the balls 4, 4 held in the same pocket 10c, thereby ensuring a durability of the cage 9b. 
In the case of the above-mentioned structure disclosed in the specification of British Patent 1,537,067, no consideration is made with respect to the strength of the cage.
Further, as explained above, it is necessary for attaining the downsizing and the reduction in weight of the wheel-support rolling bearing unit known as the so-called fourth-generation hub unit to reduce, as shown in FIG. 25, a major diameter of a housing unit 3A by decreasing the major diameter of each of the balls 4, 4 constituting the constant velocity joint 1a and thus decreasing the diameter of the circumscribing circle of the balls 4, 4. Then, there is a necessity for ensuring a load capacity of the constant velocity joint 1a by reducing the major diameter of each of the balls 4, 4 and increasing the number of the balls 4, 4 (from 6 to 7 or more).
If the major diameter of each of the balls 4, 4 is set too small, however, there might decrease a contact ellipse existing at impingement portions between the rolling surfaces of these balls 4, 4 and the inner surfaces of the inner engagement groove 7a and the outer engagement groove 8a, with the result that a surface pressure upon those impingement portions becomes excessively large. The rolling fatigue life-span of the inner surface of each of the engagement grooves 7a, 8a is thereby shortened. If the major diameter of each of the balls 4, 4 is simply increased for preventing the reduction in the rolling fatigue life-span of the inner surface of each of the engagement grooves 7a, 8a due to the above cause, the interval between the balls 4, 4 adjacent to each other in the circumferential direction is narrowed. Then, there is decreased the width of each of the column members existing between the pockets 10, 10 for holding the balls 4, 4 with respect to the cage 9. The reduction in the width of the column member is not also preferable because of leading to the decline of the durability of the cage 9.
If the major diameter of the housing unit 3A is increased, the major diameter of each of the balls 4, 4 is also increased, and besides the width of each column member can be ensured. It is, however, impossible to attain the downsizing and the reduction in the weight of the wheel-support rolling bearing unit called the fourth-generation hub unit, which is not preferable.
It is a first object of the present invention to provide a constant velocity joint that can be downsized and reduced in weight, and is capable of transmitting a sufficient torque.
It is a second object of the present invention to provide a rolling bearing unit for a wheel, which can be downsized and reduced in weight.
According to a first aspect of the present invention, a constant velocity joint comprises an inner race, inner engagement grooves each taking a circular arc in section and formed in eight locations at an equal interval in a circumferential direction on an outer peripheral surface of the inner race in a direction right-angled to the circumferential direction, an outer race provided along a periphery of the inner race, outer engagement grooves each taking a circular arc in section and formed in positions facing to the inner engagement grooves on an inner peripheral surface of the outer race in the direction right-angled to the circumferential direction, a cage sandwiched in between an outer peripheral surface of the inner race and an inner peripheral surface of the outer race and formed with eight pockets each elongated in a circumferential direction in positions aligned with the inner engagement groove and the outer engagement groove, and eight pieces of balls made capable of rolling along the inner engagement groove and the outer engagement groove in a state of being singly held inwardly in each of the pockets. Then, a crossing angle between a central axis of the inner race and a central axis of the outer race is bisected, and the balls are disposed within a bisection plane orthogonal to a plane including these two central axes.
Particularly in the constant velocity joint according to the first aspect of the present invention, if a ratio tc/Da is set to rt, there is satisfied a relationship such as:
(0.054/rt)xc2x7Daxe2x89xa6wxe2x89xa6(0.16/rt)xc2x7Da 
where Da is the major diameter of each ball, w is the circumference-directional width of each of the column members existing between the pockets adjacent to each other in the circumferential direction with respect to the cage, and tc is the diameter-directional thickness of each of the column members of the cage.
According to the thus structured constant velocity joint in the first aspect of the present invention, it is feasible to sufficiently ensure both of the rolling fatigue life-span of each of the inner and outer engagement grooves and the strength of the cage, and the constant velocity joint is downsized, thus making a contribution to utilization of a so-called fourth generation hub unit in which the outer race of the constant velocity joint is integrated with the inner race of the rolling bearing unit for supporting the wheel.
Further, in the constant velocity joint according to a second aspect of the present invention, if a ratio Dc/dm of a diameter Dc of the outer peripheral surface of the cage to a pitch circle diameter dm of each of the plurality of balls is set to R1, and if a ratio dc/dm of a diameter dc of the inner peripheral surface of the cage to a pitch circle diameter dm is set to r1, there are relationships such as 1.06 less than R1 less than 1.11, and 0.945 less than r1 less than 0.998. Note that the pitch circle diameter dm is a 2-fold dimension of a distance between a center (a point Oi or Oe in FIGS. 6-8 and 21) of curvature of a bottom surface of the inner or outer engagement groove and a center of each ball when the constant velocity joint is in a neutral state (wherein a joint angle is 0).
Further, in the constant velocity joint according to the second aspect of the present invention, in addition to the preferable construction given above, a ratio rt of an average thickness tc of the cage which is expressed by xc2xd of a difference between a diameter Dc of the outer peripheral surface of the cage and a diameter dc of the inner peripheral surface of the cage, to a major diameter Da of each ball, that is, rt=tc/Da has a relationship such as 0.16 less than rt less than 0.30.
According to the thus constructed constant velocity joint in the second aspect of the present invention, even in the case where for instance, the width of the column member provided between the pockets adjacent to each other is decreased by setting the number of the pockets for holding the balls to 8, a sectional area of each column member is secured, whereby a strength and a durability of these column members can be ensured. Simultaneously, it is feasible to prevent an opening peripheral edge of the pocket from being chipped by preventing the rolling surface of each ball from impinging upon the opening peripheral edge of the pocket.
To start with, there will be elucidated the reason why the ratio Dc/dm (=R1) of the diameter Dc of the outer peripheral surface of the cage to the pitch circle diameter dm of each of the plurality of balls is set such as 1.06 less than R1 less than 1.11.
When designing the constant velocity joint, the pitch circle diameter dm is determined as a principal item of data corresponding to the number of the balls and the major diameter Da in order to obtain a load capacity corresponding to a magnitude of the torque to be transmitted. Accordingly, the above ratio Dc/dm (=R1) is a value determined depending mainly upon a magnitude of the diameter Dc of the outer peripheral surface of the cage.
As it becomes more approximate to R1xe2x89xa61.06, the diameter Dc of the outer peripheral surface of the cage becomes smaller, in which case the sectional area of the cage is ensured with a difficulty, and besides the rolling surface of each ball might impinge upon an opening peripheral edge on the side of the major diameter of the pocket, with the result that this peripheral edge is easily chipped off. The impingement of the rolling surface upon the peripheral edge can be prevented by reducing an offset quantity (shown by h in FIG. 6) of points of centers of curvature of the bottom surfaces of the two inner and outer engagement grooves and thus decreasing a displacement quantity of each ball in the diametrical direction of the cage. The reduction in the offset quantity, however, is a cause for hindering a smooth operation of the constant velocity joint and can not be therefore adopted.
In contrast, as it becomes more approximate to R1xe2x89xa61.11, the diameter Dc of the outer peripheral surface of the cage becomes larger, in which case a depth of each outer engagement groove becomes too small. Then, as explained above, the rigidity of the constant velocity joint in a rotational direction lowers, and the rolling surface of the ball becomes easier to run on the side edge of the opening of each of the outer engagement grooves. It is consequently difficult to ensure the durability of the constant velocity joint such as a shortened rolling fatigue life-span of the rolling surface of each ball.
According to the second aspect of the present invention, R1 being set such as 1.06 less than R1 less than 1.11, the impingement of the rolling surface upon the peripheral edge of the pocket, which might lead to the chip-off, is prevented while securing the load capacity and the smooth operation of the constant velocity joint. In addition, it is possible to prevent the rolling surface of each ball from running on the side edge of the opening of the outer engagement groove.
Given next is an elucidation of the reason why the ratio dc/dm (=r1) of the diameter dc of the inner peripheral surface of the cage to the pitch circle diameter dm is set to 0.945 less than r1 less than 0.998.
As explained above, the pitch circle diameter dm is determined as the principal item of data of the constant velocity joint, and hence the above ratio dc/dm (=r1) is a value determined depending mainly upon a magnitude of the diameter dc of the inner peripheral surface of the cage.
To begin with, as it becomes more approximate to 0.945xe2x89xa7r1, the diameter dc of the inner peripheral surface of the cage becomes smaller. In this case, the depth of the inner engagement groove becomes too small, and, as described above, the rigidity of the constant velocity joint in the rotational direction lowers, and besides the rolling surface of each ball becomes easier to run on the side edge of the opening of the inner engagement groove. It is consequently difficult to ensure the durability of the constant velocity joint such as a shortened rolling fatigue life-span of the rolling surface of each ball.
In contrast, as it becomes more approximate to r1xe2x89xa70.998, the diameter dc of the inner peripheral surface of the cage becomes larger. In this case, it is difficult to ensure the sectional area of the cage, and additionally the rolling surface of each ball impinges upon the peripheral edge of the opening on the side of the minor diameter of the pocket, with the result that this peripheral edge is easily chipped off. As explained above, it is unfeasible to reduce the offset quantity to prevent the impingement of the rolling surface of each ball on the peripheral edge of the opening.
According to the second aspect of the present invention, r1 being set such as 0.945  less than r1 less than 0.998, the impingement of the rolling surface upon the peripheral edge, which might lead to the chip-off, is prevented while securing the load capacity and the smooth operation of the constant velocity joint. In addition, it is possible to prevent the rolling surface of each ball from running on the side edge of the opening of the outer engagement groove.
Furthermore, the ratio rt(=tc/Da) of the average thickness tc of the cage which is expressed by xc2xd of the difference between the diameter Dc of the outer peripheral surface of the cage and the diameter dc of the inner peripheral surface of the cage to the major diameter Daof each ball, is set such as 0.16 less than rt less than 0.30, it is possible to give a well equilibrium between the strength of each column member and the durability of each of the two groups of inner and outer engagement grooves in the constant velocity joint as a whole.
Namely, as it becomes more approximate to rt less than 0.16, the average thickness tc of the cage becomes smaller as compared with the major diameter Da of each ball, in which case there diminishes the sectional area of the column member existing between the pockets adjacent to each other in the circumferential direction, and the strength and the durability of the cage including the column members are secured with the difficulty.
By contrast, as it becomes more approximate to rtxe2x89xa70.30, the average thickness tc of the cage becomes larger as compared with the major diameter Da of each ball. In this case, the depth of each of the inner and outer engagement grooves is hard to ensure, resulting in a difficulty of transmitting the large torque.
That is, as obvious from the explanation of FIGS. 6-18, the depths of the inner and outer engagement grooves are not uniform over their entire length, and each groove becomes deep at its one end in the lengthwise direction but shallow at the other end. In order that the large torque can be transmitted (the sufficient load capacity is ensured) by the thus structured rzeppa type constant velocity joint, and besides, the enough durability is secured, it is required that the depth of each of the inner and outer engagement grooves be amply secured at each of the other ends having the smallest depths. On the other hand, if the offset quantity h described above augments for ensuring the smooth operation of the constant velocity joint, a difference in the depth between the inner engagement groove and the outer engagement groove becomes large between one end and the other end. In such a state, when the average thickness of the cage is increased while reducing the depths of the inner and outer engagement grooves, it is difficult to secure this depth at each of the other ends where the depths are minimized.
In contrast, if the ratio rt is set such as 0.16 less than rt less than 0.30, it is feasible to establish a compatibility between ensuring the strength and the durability of the cage including the column members and securing the load capacity of the constant velocity joint which is based on ensuring the depths of the two engagement grooves.
According to the above-described second aspect of the present invention, the constant velocity joint that is excellent of its rigidity and durability can be obtained.
According to the third aspect of the present invention, at least a part of the plurality of pockets are capable of holding the plurality of balls within the single pocket, and a total number of balls is 7 or more (preferably 8 or more).
According to the third aspect of the present invention, the number of the pockets is preferably even-numbered, and the number of the balls held in each of the pockets is different.
According to the third aspect of the present invention, the number of the pockets is preferably 4, and the number of the balls is 10. The number of the balls held in each of the two pockets existing on the opposite side in a diametrical direction is 2, and the number of the balls held in each of the remaining two pockets is 3, respectively.
In the case of the thus constructed constant velocity joint according to the third aspect of the present invention, the total number of the balls is set to 7 or more, and therefore, even when capable of transmitting sufficiently a large torque or the dimension of the major diameter is decreased, the constant velocity joint can be downsized and reduced in weight. Besides, an interval between the balls held in the same pocket is narrowed, and a length dimension of a column member existing between the pockets adjacent to each other in the circumferential direction is increased, so that the length of the column member may be 4, 5 or 6 mm in average, for example, and the thickness in the diameter direction may be 3.2, 3.5, 3.8 or 4.1 mm, for example, thus ensuring a durability of the cage and obtaining an enough durability of the constant velocity joint as a whole.
In particular, according to the preferable characteristics of the constant velocity joint in the third aspect of the present invention, when the number of the balls held in each pocket is made different, the balls are incorporated finally into the pockets in which a larger number of balls should be held, thereby making it feasible to incorporate the balls and, besides, to restraining an increased in the length dimension of the pocket. As a result, the number of the balls to be incorporated into the constant velocity joint can be increased, and in addition the durability of the cage can be ensured.
According to a fourth aspect of the present invention, a rolling bearing unit for a wheel, comprises an outer race constructive member including respectively a first fitting flange, formed on an outer peripheral surface, for supporting the outer race constructive member on a suspension, and a plurality of outer race tracks formed on an inner peripheral surface, the outer race constructive member not rotating when used, an inner race constructive member including respectively a second fitting flange, formed on an end side portion of an outer peripheral surface, for supporting a wheel, and a plurality of inner race tracks formed on an intermediate portion, the inner race constructive member having the other side end portion formed as a housing unit serving as an outer race of a constant velocity joint, the inner race constructive member rotating when used, a plurality of rolling members so provided as to be capable of rolling between the outer race tracks and the inner race tracks, an inner race provided on an inner side of the housing unit and constituting the constant velocity joint, outer engagement grooves each taking a circular arc in section and formed in a direction right-angled to a circumferential direction in a plurality of positions in the circumferential direction on an inner peripheral surface of the housing, a plurality of inner engagement grooves each taking a circular arc in section and formed in the direction right-angled to the circumferential direction in positions facing to the outer engagement grooves on an outer peripheral surface of the inner race, a cage sandwiched in between an outer peripheral surface of the inner race and an inner peripheral surface of the housing unit and formed with a plurality of pockets each elongated in the circumferential direction in positions aligned with the inner engagement grooves and the outer engagement grooves, the cage constituting the constant velocity joint, and a plurality of balls so provided as to be capable of rolling along the inner engagement grooves and the outer engagement grooves between the outer engagement grooves and the inner engagement grooves in a state of being held inwardly of the pockets. A crossing angle between a central axis of the inner race and a central axis of the housing unit is bisected, and the balls constituting the constant velocity joint are disposed on a bisection plane orthogonal to a plane including these two central axes.
Especially in the rolling bearing unit for the wheel according to the present invention, the number of the outer and inner engagement grooves and the number of balls are each set to 7 or more. Further, a radius of curvature of each of sectional configurations of the two groups of outer and inner engagement grooves is made small at each of groove bottom areas of the two groups of outer and inner engagement grooves and made larger at both of side end portions proximal to respective opening edges. Moreover, if a ratio d1/d2 of d1 to d2 is set to R, there is a relationship such as:
0.49xe2x89xa6Rxe2x89xa60.63 
where d1 is the pitch circle diameter of each of the balls constituting the constant velocity joint, and d2 is the pitch circle diameter of each of the rolling members constituting the inner rolling member train of the plurality of rolling member trains.
In the case of the rolling bearing unit for the wheel according to the fourth aspect of the present invention, a contrivance is given to each of the sectional configurations of the two groups of inner and outer engagement grooves, and there is controlled the ratio R(=d1/d2) of the pitch circle diameter d1 of each of the balls constituting the constant velocity joint to the pitch circle diameter d2 of each of the rolling members constituting the inner rolling member train of the plurality of rolling member trains of which the rolling bearing unit is constructed. The wheel-support rolling bearing unit can be thereby downsized and reduced in its weight while ensuring the durability of this wheel-support rolling bearing unit.
Namely, the radius of curvature of each of the sectional configurations of the two groups of outer and inner engagement grooves is made smaller at each of the groove bottom areas of these two groups of engagement grooves and larger at both of the side ends proximal to the respective opening edges. It is therefore feasible to increase a contact angle between the inner surface of each of the two groups of engagement grooves and the rolling surface of each ball. Hence, in combination with the contrivance that the number of the balls is set to over 7 larger than 6 set according to the prior art, the load capacity of the constant velocity joint augments, and the durability of this constant velocity joint can be thereby ensured. Further, the balls run on the shoulder portions of the two groups of outer and inner engagement grooves with the difficulty, thereby preventing an excessive surface pressure based on an edge load from acting on the rolling surface of each ball. Consequently, an exfoliation life-span of the rolling surface of the ball can be ensured.
Further, when the above ratio R is controlled within a range such as 0.49xe2x89xa6Rxe2x89xa60.63, in a case where the number of the balls constituting the constant velocity joint is set to 7 or more, the major diameter of each of the balls is secured, and there is enlarged a contact ellipse existing at an impingement portion between the rolling surface of each of the balls and an inner surface of each of the two groups of outer and inner engagement grooves, thereby restraining a rise in a contact surface pressure of the above impingement portion. It is therefore possible to ensure the rolling fatigue life-span of the inner surface of each of the two groups outer and inner engagement grooves. Simultaneously, the durability of the cage can be secured by ensuring the width of each of the column members existing between the pockets for holding the balls with respect to the cage. Besides, there can be made such a design that a part of the outer engagement grooves are disposed on the inner side in the diametrical direction of the inner rolling member train, whereby the wheel-support rolling bearing unit known as the fourth-generation hub unit can be downsized and reduced in its weight.
Note that as the ratio R becomes less than 0.49, the pitch circle diameter d1 of the ball decreases. Then, the interval between the balls adjacent to each other in the circumferential direction is narrowed, and the width of the column member diminishes, with the result that the durability of the cage can not be ensured. Further, the interval between the balls adjacent to each other in the circumferential direction is increased in order to ensure the durability of this cage, and hence, if the major diameter of each ball is decreased, the contact surface pressure rises, it is difficult to secure the rolling fatigue life-span of the inner surface of each of the two groups of outer and inner engagement grooves.
By contrast, as the ratio R exceeds 0.63, the pitch circle diameter d1 of the ball increases. Then, it is difficult to make such a design that a part of the outer engagement grooves are disposed on the inside in the diametrical direction of the inner rolling member train, and the wheel-support rolling bearing unit known as the fourth-generation hub unit is downsized and reduced in its weight with the difficulty.
Other features and advantages of the present invention will become readily apparent from the following description taken in conjunction with the accompanying drawings.